Various techniques and apparatus have been used in attempts to separate the mill starch fraction of a corn wet milling process into its major protein and starch components in such a way that recovery of these components would be both economical and of commercially acceptable quality.
The techniques and apparatus used have included first concentrating the mill starch fraction and then subjecting it to multiple separation stages employing mechanical centrifugal separators such as those commercially available under the trademark "Merco" and commonly referred to as "Merco centrifugers". This approach was not entirely satisfactory as these centrifugal separators are relatively large, involve a high capital expenditure, require significant maintenance down time and operational adjustment, have a relatively high energy demand and operate most economically upon materials which have higher densities and, consequently, higher dry substance levels than those normally obtained from the mill starch stream.
Merco centrifugal separators were also used in combination with hydrocyclones, such as those commercially available under the tradename "DorrClone". The use of hydrocyclones for this and other purposes is known. For example, U.S. Pat. No. 2,689,810 discloses the use of hydrocyclones to enhance starch/gluten separation and describes a typical hydrocyclone; U.S. Pat. Nos. 3,029,168 and 3,072,501 disclose the use of hydrocyclones in a root starch isolation process; and U.S. Pat. No. 2,642,185 discloses the use of hydrocyclones to separate starch particles of different sizes. Although the combination of centrifugal separators and hydrocyclones represented an improvement over the use of only centrifugal separators, it was still a capital intensive and relatively costly operation.
It was believed that these problems could be overcome by utilizing a system containing only hydrocyclones. Such a system was put into operation from which there was obtained the protein-rich product on a 63-64% protein, insoluble dry solids basis (IDSB), whereas the minimum commercially acceptable protein level in the protein-rich product is generally recognized to be 67% IDSB. Consequently, the 63-64% IDSB protein product had to be further processed to obtain a commercially acceptable product. Substantially the same results were obtained in connection with the starch-rich product which contained about 0.45% insoluble protein IDSB whereas the generally acceptable level is less than about 0.38% insoluble protein IDSB. To obtain commercially acceptable starch-rich and protein-rich products, therefore, both of these products had to be subjected to further treatment making the system economically unattractive.
Thus, these prior art approaches utilizing only hydrocyclones have made it possible to obtain separation of either a starch-rich fraction or a gluten-rich fraction, but it has not been possible to obtain concurrent separation of both fractions that meet commercially acceptable levels. In particular, U.S. Pat. No. 2,689,810 (mentioned above) discloses an all hydrocyclone system and indicates that this system can be used to concurrently separate both a starch-rich fraction and a gluten-rich fraction. Supposedly, both fractions would be at commercially acceptable levels. However, close scrutiny of this patent reveals that there is no teaching of how many separation stages are to be employed, how many hydrocyclones are to be used in the separation stages, whether the hydrocyclones used can be of different size or must be the same size, whether it is the actual pressures used in the system or the pressure differentials or both that are important, and whether the concentration of the gluten(protein)-rich fraction is important.
According to the disclosure of U.S. Pat. No. 2,689,810, a system utilizing only hydrocyclones was employed to obtain a starch containing 0.28%-0.32% protein (Col. 7) and a gluten fraction containing 60%-70% protein (Col. 8). Attempts to concurrently obtain these products based upon Example D of this patent were not successful. Example D of U.S. Pat. No. 2,689,810 indicates a gluten stream having a concentration of 15-20 grams/liter (2.0-2.6 ounces/gallon) of insoluble dry substance. However, it was found that gluten concentration, as well as the pressures used in the system, are critical if commercially acceptable starch and protein products are to be concurrently obtained.
Thus, while it is possible to obtain either a commercially acceptable starch or protein product by using the system and conditions suggested in U.S. Pat. No. 2,689,810, it has not been possible to obtain both products concurrently at commercially acceptable levels. Unless this can be achieved, such a system would be economically unattractive.